1. Field of the Invention
The present invention relates to electrical signal processing, and, in particular, to mixers used for frequency conversion.
2. Description of the Related Art
A mixer is a device that converts electrical signals from one frequency range to another frequency range. Series of two or more mixers are typically used in radio architectures, for example, during transmit processing, to convert signals from a baseband frequency range to an intermediate frequency (IF) range (e.g., 50-500 MHz) and from the IF range to a radio frequency (RF) range. In addition, series of two or more mixers may be used, during receive processing, to convert signals from the RF range to the IF range and from the IF range to the baseband frequency range.
FIG. 1 shows a schematic diagram of a prior-art mixer circuit 100 used to convert an input voltage signal Vin in an input frequency range into an output voltage signal Vout2 in an output frequency range. Mixer circuit 100 consists of a sequence of two mixer stages, where each mixer stage has a voltage-to-current (V-I) converter 102, a mixer core 104, and a current-to-voltage (I-V) converter 106.
V-I converter 102a converts the input voltage signal Vin into a current. Mixer core 104a mixes (i.e., multiplies) the current from converter 102a with a first local-oscillator signal LO1, and I-V converter 106a converts the resulting current into an intermediate voltage signal Vout1. Similarly, V-I converter 102b converts the intermediate voltage signal Vout1 into a current, mixer core 104b mixes the current from converter 102b with a second local-oscillator signal LO2, and I-V converter 106b converts the resulting current into the output voltage signal Vout2.
One of the problems with mixer circuit 100 is that each step of converting signals from voltage to current adds distortion to the signal stream that may ultimately adversely affect the fidelity of the information carried by the signal stream. Another problem is that each mixer stage usually adds gain, so that each successive voltage-to-current conversion must consume more power than the previous conversion in order to maintain the same linearity.
FIG. 2 shows a schematic diagram of an alternative prior-art mixer circuit 200 designed to perform signal-frequency conversion. The design of mixer circuit 200 is based on a folded mixer stage in which the V-I converter 202 is separated from the first mixer core 204. The current generated by V-I converter 202 is AC coupled through capacitors Cc into the emitter nodes (or equivalent) of the mixer core 204. The capacitors Cc block the DC bias current and allow only the AC signal current to be coupled. To work properly, the load impedance of the V-I converter 202 must be high relative to the sum of the coupling capacitor impedance and the impedance looking into the emitter nodes of the mixer core 204. The current divides through current division into the two impedances and the majority will flow into the mixer core 204, if the low-impedance requirement has been met.
If the load impedances of mixer core 204 are sufficiently high relative to the emitter nodes (or equivalent) of the second mixer core 206, the signal current will be delivered to the second mixer core 206 by current division in a fashion similar to that of the first mixing stage. This current division concept may be extended to additional mixing stages if desired.
The low-impedance requirement sets a minimum size limit on the capacitors Cc. The maximum size of the capacitors is determined by the value of the parasitic capacitance Cp of the coupling capacitor. After the signal current flows through the coupling capacitor Cc, it will further divide between the desired path into the mixer core 204 and the substrate through the parasitic capacitance Cp. To ensure that the majority of the current flows into the mixer core 204, the impedance seen looking into the parasitic capacitance Cp must be large compared to the impedance looking into the emitter nodes. This sets a maximum size limit on the parasitic capacitance Cp, and, since its size is proportional to the value of the coupling capacitor Cc, this also bounds the size of the coupling capacitor Cc. The bounds on the capacitor size limit the use of mixer circuit 200 to frequencies where the capacitor can practically be made on chip.
A goal of the present invention is to overcome the limitations of the prior art Further aspects and advantages of this invention will become apparent from the detailed description which follows.